1. Field of the Invention
The present invention relates to the powering of a plurality of functional circuits within a telephone or other terminal device, through a subscriber's line of a telecommunication system, such as a telephone network, and, in particular, to a system for splitting a limited amount of supply current received from the line among a plurality of functional circuits in a terminal device, such as a telephone, capable of being powered through the line.
2. Discussion of the Related Art
In certain telecommunication networks and typically in wired telephone systems, a plurality of functional circuits, e.g. the speech circuitry, the dialling circuit, etc. of a telephone, must be capable of functioning with electrical power made available through the subscriber line by a dedicated battery installed in a remote switching station. Commonly, within an apparatus connectable through a user's line to a central system, there is a circuit dedicated to oversee the derivation from the line of a DC current necessary for powering the various functional circuits at respective stabilized supply voltages.
In a prior U.S. patent application Ser. No. 07/991,564, filed on Dec. 16, 1992, by the present applicant, an improved stabilized voltage power supply circuit for functional circuits of a subscriber's apparatus is disclosed. The circuit derives a current from the line and is capable of ensuring a reduced voltage "droop", i.e. a low voltage drop along the path between the physical connection point of the apparatus to the line and the point where a certain current is absorbed at a stabilized voltage by a certain functional circuit of the subscriber's apparatus. The bifilar subscriber's line normally constitutes also a signal path in these telecommunication systems. An important aspect of the practice of powering several user's circuits through a bifilar line, which has a nonnegiigeable influence on the global power consumption of the system, is represented by the manner in which a given amount of current that may be derived from the subscriber's line (normally a major portion of it) is eventually split among a plurality of functional circuits to be powered in the apparatus. The splitting must necessarily take place in respect of a certain predefined order of priority or rank of the various circuits. A typical example of this recurrent situation is represented by the subscriber's side speech circuits, where a dedicated circuit present in the telephone derives a current from the telephone line and distributes it, partially for powering itself, partially for powering logic control circuits, partially for powering an eventual microprocessor, partially for powering a circuit overseeing the so-called amplified-listening mode (if present in the telephone) and so forth among a plurality of functional circuits. Each of these functional circuits may have a maximum design current absorption and, in such an operating range, it may have a variable instantaneous current absorption depending on the actual working conditions. Depending on the functional characteristics and conditions of the various circuits and the interactions among the circuits, a certain rank of priority is assigned to each circuit and is used to determine the possibility of that circuit receiving power via the circuit overseeing the splitting of the available DC current derived from the line.
Commonly, in normal systems, the subdivision of the available supply current is performed taking into account the rank of priority of the various functional circuits, in the sense that current is supplied first to the circuit ranking higher in priority and so forth to all the other circuits in a decreasing order of priority as far as there is an adequate availability of supply current. For each user's circuit a maximum current absorption is set at the design stage and according to the known technique each of them will absorb in practice such a maximum current whenever they are activated. In other words, the available line current, that is the current which is derived by a certain subscriber I.sub.L, is split so as to supply current to the functional circuit of highest rank for a maximum current absorption of that particular circuit. The remaining current is made available to a second functional circuit of a lower rank for its maximum design current absorption value and the remaining current may then be made available to a third functional circuit of lower rank and so forth. This common system of splitting the available supply current among a plurality of functional circuits of different rank, may be expressed by the relationship: EQU I.sub.L =I1max+I2max+I3max+. . .
where I.sub.L is the total current which is derived from the line.
This way of managing the splitting of the supply current is not always efficient. For example, if the current which is actually absorbed by a certain functional circuit of the apparatus connected to the line has a relatively low stand-by value (DC) and relatively large absorption peaks (AC), a remarkable waste of current may occur.